Melinda, Try having students cover all the colors with a thick coat of white crayon before painting over. I use India ink instead of paint. I tell them to color heavy with their colors and that the white should look like a thick fog over their colors. I have students use sharpened pencils to scratch - safer than paper clips and they don't break as easily as toothpicks. Good Luck Linda W.
For subject matter I have used city scapes, outer space,
dinosaurs, and jungles - depends on grade level and classroom connection if possible. L
Problem: the color does not come through well after scratching.
also, what kind of subject matter lends itself to this activity